PHYSICAL EXPLANATION FOR MULTIPLE-CELL CLASSES AFTER CENTRIFUGATION IN COLLOIDAL SILICA GRADIENTS

When cells of Dictyostelium discoideum were centrifuged to density equilibrium in linear gradients of colloidal silica (Ludox), approximately 40 discrete bands appeared. This suggested that there could be at least 40 different cell types. A similar result was found for formalinized red blood cells [sheep] and plastic beads. Isolated bands of cells rebanded faithfully in new gradients, and band spacing depended upon gradient steepness. However, the cell bands resulted from microscopic modifications in the linear gradients of Ludox caused by centrifugation. When the gradients were analyzed in the analytical ultracentrifuge, absorbance scans revealed that the cell bands coincided with bands of Ludox, which formed even without cells. Evidence ruling out other possible causes for cell bands is presented. Additionally, procedures which avoid this condition are described.